Well the title says it all..

Many new players think 20 is a marvelous size for Men at arms (M@A).
Probably because the Battalion box comes with 20 of them.

Here is the explaination why it is not a good unitsize.   

Why is it so stupid to make a unit of M@A with full command exactly 20 big..?
Well one wound from shooting or magic will lower the 3 ranks you have paid for instantly to 2 full ranks..
Also getting the outnumbering bonus in close combat will be very doubtfull.

And M@A need full ranks and outnumbering bonuses to win the combat..
If  they actually manage to kill  something in combat, it is a time for much rejoicing;) Don't count on it..

M@A need fullranks, banner, musician and a shot at the outnumbering bonus to have any chance of winning a combat..

+5CR and a 4+save makes it a decent battleunit

So use units of about 25 if you want them to hold ground with them..
those 25 extra points investment make it a far better unit....

If you want units that are there as bait, use units of about 10, the minumum unitsize you can buy, perhaps with a musician for those rally test..(if they survive the battle you can even claim table quarters with them:) Invest 55 points get 100 back)

That are the 2 unitsizes that work quite well.

Small support units of 10 and big battle units of 25..

Don't have enough models to field a unit of 25?

Archers model could be easily build as M@A without shield..
mix a few of them in the formation.. Or even unconverted models :)